Anything up to about 35 feet LOA is easily manageable with a tiller in all but the nastiest conditions. If your 30-foot boat seems too big to handle with a tiller, you probably need to reduce sail and/or balance the sails. Cal 40's originally came with tillers, although they were far from cruising boats. As others have said, a tiller will give the helmsman far more feedback making it far easier to really be "in tune" with the boat. A tiller allows much faster adjustments, particularly when operating in tight quarters (such as docking). Additionally, a tiller is simpler (always a plus), cheaper (another plus), and tilts up and out of the way when not sailing.
But I find the biggest advantages to a tiller are: the ability of the helmsman to steer just about anywhere in the cockpit using a tiller extension; and being able to steer by straddling the tiller (or just hooking a leg over the tiller whilst seated), thus leaving both hands free to trim sails, untangle lines
, operate the radio
, eat a snack, open a beer, et cetera.